In rotary combine harvesters, there is a driven main rotor to which the crop is fed. The main rotor can be subject to plugging which occurs when the main rotor becomes jammed with crop because the crop is fed thereto in larger quantities than can be passed by the main rotor. The control of this invention senses the pressure in the closed circuit of the hydrostatic transmission and, when the pressure reaches a predetermined value, the control operates to adjust the variable displacement unit of the hydrostatic transmission to increase the speed of the main rotor. It is known in the art to sense the pressure in the closed circuit of a hydrostatic transmission and, at some level of pressure, cause a change in the displacement of the variable displacement unit. However, such a control has been to reduce the torque applied to the motor, rather than cause a change in the speed of the motor of the transmission to increase the speed of the working element driven by the transmission.